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Light Sensor

Publishing Venue

IBM

Related People

Masetti, M: AUTHOR

Abstract

The circuitry senses light variations and produces an output signal. Its rise time does not depend upon the rate of variation of light intensity. The system comprising a photosensitive element serially connected to a tunnel diode switches a transistor every time the light conditions on a photosensitive element pass from dark to light state and vice versa.

Country

United States

Language

English (United States)

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Light Sensor

The circuitry senses light variations and produces an output signal. Its rise
time does not depend upon the rate of variation of light intensity. The system
comprising a photosensitive element serially connected to a tunnel diode
switches a transistor every time the light conditions on a photosensitive element
pass from dark to light state and vice versa.

Readout photocell C has the current-voltage characteristics a(1), a(2), a(3),
a(4), a(5) as a function of increasing light intensity. The intersections of these
characteristics with the tunnel diode characteristic b presents the working point of
the solar cell, tunnel diode system.

Point A is the working point in the dark state. Under this condition, transistor
T is cut off. By increasing the light intensity, the working point shifts from position
A toward point B. When this is reached, the tunnel diode switches along its
characteristic b and the operating point of the system becomes B'. This varies the
base voltage of T, which rapidly switches into saturation. When the light intensity
decreases, a rapid switching of T to cutoff occurs.

The output signal rise does not depend upon the variation speed of light
intensity. This is because variation speed is very slow compared to the switching
speed of the system. For this reason, it is sufficient that the light intensity reach
the characteristic a5 level to obtain a rapid switching of the system.